Fluid tunnels are used in a variety of applications to measure the effects of fluid flow over an object. For example, in aerodynamic testing, wind tunnels are used to measure the response of a test article such as a scale model aircraft to air passing over the aircraft. The wind tunnel provides a means to evaluate the test article in a controlled environment under conditions that are dynamically similar to conditions to which a full-size version of the aircraft may be subjected in actual flight. Air in the wind tunnel flows over the scale model aircraft at a controlled speed in order to evaluate the aerodynamic response of the scale model aircraft at different positions and attitudes. The attitude of the scale model aircraft may be defined in terms of roll, pitch, and yaw about the respective longitudinal, lateral, and vertical axes of the aircraft.
Scale model aircraft in a wind tunnel may be supported by a variety of different means including, but not limited to, a sting and balance mechanism extending out of an aft end of the aircraft. The sting forms part of a support mechanism for the aircraft and provides the ability to statically position or dynamically move the aircraft. For example, in changing the pitch angle of the aircraft, the sting may rotate the aircraft from a current attitude (e.g., a zero-pitch attitude or horizontal attitude) to a positive pitch attitude wherein the forward end of the aircraft is pointing upwardly relative to horizontal. In the positive pitch attitude, the wings of the aircraft may be oriented at a positive angle of attack relative to the direction of fluid flow through the wind tunnel. The direction of fluid flow may be assumed to be generally parallel to the tunnel walls or to the tunnel centerline which may also be assumed to be horizontal.
The support mechanism or sting may be commanded to change the roll attitude of the aircraft (i.e., rotation about a longitudinal axis of the aircraft) or yaw attitude (i.e., rotation about a vertical axis). The above-noted changes in roll, pitch and yaw as well as positional changes (i.e., horizontal or vertical movement) may be performed in order to measure air loads on the aircraft at different attitudes and positions. Three main forces that act on the aircraft include lift, drag and side force. As is known in the art, lift is a force that is directed vertically upwardly, drag force acts in an axially aft direction, and side force acts in a sideways or lateral direction. Other loads induced on the aircraft as a result of fluid flow include rolling moment, pitching moment and yawing moment which act as a torque on the aircraft about the respective longitudinal, lateral and vertical axes of the aircraft.
Measurement of the above-noted forces may be facilitated using strain gauges or other instrumentation mounted on the balance which may be integrated into the aft-mounted sting that supports the aircraft. The strain gauges measure the magnitude of forces such as lift and drag that act on the aircraft at different attitudes and positions. Accurate determination of the magnitude of the force on a scale model aircraft is necessary in order to develop and improve the design configuration of a full size aircraft. As is known, minor reductions in aerodynamic drag acting on an aircraft such as a commercial airliner can result in a significant reduction in fuel consumption when measured over long distances.
Included in the prior art are several methods for measuring the attitude of an object such as an aircraft in a wind tunnel. For example, mechanical instrumentation such as potentiometers mounted on the support mechanism measure attitude (e.g., pitch and roll) of the aircraft. Potentiometers or other attitude-measuring instrumentation may be used in conjunction with force-measuring instrumentation such as strain gauges. Although potentiometers are generally suitable for indicating the attitude of the aircraft, they possess certain limitations which may reduce accuracy with which attitude is measured.
For example, with the tunnel in a wind-on mode wherein fluid is flowing over the aircraft, air loads acting on the aircraft can deflect the sting which supports the aircraft. Because of their mounting arrangement, potentiometers may be incapable of measuring sting deflection which may compromise the accuracy of aircraft attitude measurements. For example, if the support mechanism is commanded to rotate the aircraft from horizontal to a 2.5 degree positive pitch angle, deflection or bending in the sting as a result of air loads on the aircraft can result in an actual attitude of less than 2.5 degrees despite an indicated 2.5 degree pitch angle reading from the potentiometer. As a result, drag force measured by the strain gauges may be incorrectly correlated to the indicated 2.5 degree pitch angle. Accuracy of attitude measurements may further be compromised as a result of mechanical movement or hysteresis in structure connecting the support mechanism to the sting or the sting to the aircraft.
Another prior art method for measuring attitude of an object in a wind tunnel includes the use of photogrammetry or videogrammetry wherein a system of cameras are mounted in spaced locations in the wind tunnel. The cameras are directed at targets applied to the surfaces of the test article such as a scale model aircraft. The cameras record images of the targets to measure the effects of fluid flow over the surfaces. By combining positional data regarding the known locations of the cameras relative to the test article, known locations of the aircraft relative to the wind tunnel, and known locations of the targets relative to the aircraft, the actual attitude of the aircraft can be accurately determined using the collection of recorded images taken by the system of cameras.
Unfortunately, the use of photogrammetry or videogrammetry as described above is computationally intensive due to the large number of cameras (e.g., eight cameras) required and the need to calibrate the positions of the cameras relative to the wind tunnel. Furthermore, photogrammetric and videogrammetric applications lack the ability for real-time visualization of the test article during wind tunnel operation. Real-time visualization of the aircraft is desirable in order to verify that the attitude measurement system is functioning properly during testing and that measurement data is being accurately recorded in order to avoid the time and expense of repeat testing. Likewise, real-time visualization of the attitude, position and deformation of the test article or support mechanism (i.e., sting) is desirable so that alterations in the test program can be made during testing.
As can be seen, there exists a need in the art for a system for monitoring a test article in a wind tunnel which allows for highly accurate measurement of the actual attitude and position of the test article during testing. Additionally, there exists a need in the art for a system for accurately controlling the attitude and position of the test article during testing. Finally, there exists a need in the art for a system for real-time visualization of the test article during test in order to observe the response of the test article and verify that attitude measurement and data acquisition systems are functioning properly.